Diesel engines convert chemical energy from a fuel into mechanical energy. The fuel usually is petroleum-based. Most diesel engines compress air in a cylinder and then inject fuel into the cylinder for the compressed air to ignite. The ignited fuel generates rapidly expanding gases that actuate a piston in the cylinder. The piston usually is connected to a crankshaft or similar device for converting the reciprocating motion of the piston into rotational motion. The rotational motion from the crankshaft may be used to propel a vehicle, operate a pump or an electrical generator, or perform other work. The vehicle may be a truck, an automobile, a boat, or the like.
Many diesel engines use a fuel injection system to inject fuel into the cylinders. Fuel injection systems usually have fuel injectors that spray a measured amount of fuel into each cylinder. The fuel pressure typically is increased for injecting the fuel near or at the end of the compression cycle. Most fuel injectors are activated mechanically or hydraulically and may be electronically controlled. Some diesel engines use hydraulically activated electronically controlled unit injection (HEUI) fuel system to inject fuel into the cylinders. HEUI fuel injectors usually inject fuel into the cylinders at a pressure in the range of about 15,000 psi (103 MPa) through about 25,000 psi (172 MPa).
Most diesel engines have a fuel system to provide fuel to the fuel injectors. The fuel system usually has a fuel line that interconnects a fuel tank, a fuel pump, a pressure regulator, a primary filter, and a fuel rail. The fuel system may have other components between the fuel tank and the fuel pump such as a fuel heater, a coarse filter or strainer, and a water separator. The fuel system may have other components.
Most vehicles have the fuel tank outside the engine compartment. Many vehicles have the remaining components of the fuel system inside the engine compartment. Some vehicles have a filter, a pump, and other components in or near the fuel tank. Some fuel supply components like the fuel rail are mounted on or are part of the engine.
The fuel tank is connected to the inlet side of the fuel pump. In operation, the pump generates low pressure or a vacuum on the inlet side for fuel to flow from the fuel tank to the fuel pump. The fuel may pass through the fuel heater, the coarse filter or strainer, and the water separator on the way to the fuel pump. The fuel heater usually has an electric heating element to raise the fuel temperature. The fuel heater may use engine coolant to raise the temperature. The coarse filter or strainer removes large contaminants from the fuel. The water separator removes water form the fuel. The coarse filter and the water separator may be integrated into the same component.
The fuel pump provides pressurized fuel to the primary filter. The primary filter removes smaller contaminants from the fuel. The primary filter provides filtered or cleaned fuel to the fuel rail or reservoir on the engine. The pressure regulator controls fuel pressure from the outlet side of the fuel pump. The pressure regulator usually is located after the primary filter. The pressure regulator recirculates fuel to the fuel tank via a fuel return line.
The fuel return line from the pressure regulator to the fuel tank may increase the costs and complexity of the engine. The return of filtered fuel to the fuel tank may decrease the service life of the primary filter and other components in the fuel system. The filtered fuel, once mixed with the fuel in the fuel tank, usually needs to be filtered again. The fuel pump may not operate efficiently and may fail when a plugged fuel filter causes the pressure to increase on the outlet side of the fuel pump.